1. Technical Field
The present invention relates to an image forming apparatus that includes an image bearing belt that bears an image and a transfer roller that forms a transfer nip by making contact with the image bearing belt and that has a concaved portion in one part of its circumferential surface, and an image forming method for such an image forming apparatus.
2. Related Art
There are, in the field of image forming techniques for forming images upon recording material such as paper, apparatuses configured to transfer an image that has first been formed on an image bearing belt onto the recording material. For example, with an image forming apparatus disclosed in JP-A-2008-122815 (see FIG. 1 of JP-A-2008-122815), an image forming station forms an image upon a transfer belt, serving as an image bearing member, that is stretched upon multiple rollers, and the image is transferred from the transfer belt onto recording material by causing the recording material to pass through a transfer nip formed by the transfer belt and a secondary transfer roller making contact with each other.
In order to carry out the image transfer from the image bearing member to the recording material at a high transfer efficiency, it is desirable to apply a high amount of pressure to the recording material as it passes through the transfer nip. On the other hand, there is a risk that the recording material will stick to the image bearing member if a high amount of pressure is applied in such a manner. For example, the image forming apparatus disclosed in JP-A-2008-122815 employs what is known as a liquid developing technique, visualizing an electrostatic latent image using a developer that has been dispersed throughout a liquid carrier, and thus sticking of the recording material due to the residual liquid carrier occurs with ease.
Accordingly, employing a technique such as that disclosed in, for example, JP-T-2000-508280 (see FIG. 2A of JP-T-2000-508280), can be considered as a way to solve such a problem. With an image apparatus disclosed in JP-T-2000-508280, a gripper, serving as a gripping member, that can freely open and close is provided in one part of the circumferential surface of a cylindrical pressure roller (this corresponds to a transfer roller); recording material is prevented from sticking to an intermediate transfer member that makes contact with the pressure roller by the gripper gripping an end of the recording material. Note that the intermediate transfer member, which serves as an image bearing member in JP-T-2000-508280, is a roller-shaped rotating member.
There are cases where the following problem occurs when the aforementioned JP-A-2008-122815 and JP-T-2000-508280 are taken in combination with each other. With an apparatus having such a combination, a transfer belt that bears an image and a pressure roller are each driven at respective constant speeds; a transfer nip is formed by the pressure roller pressing against the transfer belt, and the image is transferred onto recording material by causing the recording material to pass through the transfer nip. Here, in order to increase the quality of the image transferred onto the recording material, it is desirable to match the speed of movement of the recording material that passes through the nip to the speed of movement of the transfer belt. This is actually because a difference in the speeds of movement causes the occurrence of abrasions in the image. This is also because the burden on the driving source of the transfer belt is increased due to a difference in the speeds of movement as described above; this leads to disturbances in the speed of movement of the transfer belt, which in turn makes it impossible to carry out the primary transfer of the image onto the transfer belt in a favorable manner.
However, there are various types of recording material, and there are cases where the speed of movement of the transfer belt and the speed of movement of the recording material are skewed from each other depending on the type of the recording material that passes through the transfer nip. The amount of skew between the speeds of movement is particularly severe in the case where the thicknesses of recording materials differ. For example, if a case where a comparatively light recording material (for example, a basis weight of 127 (g/m2)) passes through the transfer nip is compared to a case where a comparatively heavy recording material (for example, a basis weight of 310 (g/m2)) passes through the transfer nip, the speed of movement at the transfer nip is higher for the heavy material than for the light paper. Accordingly, with the aforementioned apparatus, it has been difficult to form an image in a favorable image quality onto recording papers of various types, and the apparatus has thus been problematic in terms of its general applicability.